Help with cross referencing using XMATCH and INDEX.

You're right you are close. Leaving the column/row blank in INDEX (or giving it a 0) will return the whole row/column so in this case leaving the column blank (but you must still have the comma to indicate the column parameter):

=INDEX('THHN_THWN_THWN-2'!$C$4:$N$27,XMATCH(Test!$A3,'THHN_THWN_THWN-2'!$B$4:$B$27),)

should return the whole row so then wrap it with XLOOKUP:

=XLOOKUP(Test!$B3, INDEX('THHN_THWN_THWN-2'!$C$4:$N$27,XMATCH(Test!$A3,'THHN_THWN_THWN-2'!$B$4:$B$27),), 'THHN_THWN_THWN-2'!$C$2:$N$2, "not found", 1)

But if you use this in multiple locations (or even not) I recommend you NAME those ranges. for example:
ConduitRng = 'THHN_THWN_THWN-2'!$C$2:$N$2
ConductorRng = 'THHN_THWN_THWN-2'!$B$4:$B$27
CountRng = 'THHN_THWN_THWN-2'!$C$4:$N$27
so then the formula READS like:

=XLOOKUP($B3, INDEX(CountRng, XMATCH($A3, ConductorRng),), ConduitRng, "not found", 1)

BTW since you used A3 as the conductor I am assuming B3 for the count.

and lastly you should be able to make an array output accordingly:

=XLOOKUP($B3:$B100, INDEX(CountRng, XMATCH($A3:$A100, ConductorRng),), ConduitRng, "not found", 1)